A plane making equal angles with the three edges of a trihedron.

Altshiller-Court, N. "Isoclinal Lines and Planes." §2.3 in Modern Pure Solid Geometry. New York: Chelsea, pp. 32-37, 1979.

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Weisstein, Eric W. "Isoclinal Plane." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/IsoclinalPlane.html